1. Field of the Invention
The present invention relates to a passive-type distance measuring apparatus that is provided with a distance measuring sensor which converts incident light into an analog signal.
2. Description of the Related Art
A conventional passive-type distance measuring apparatus, which is widely used as a passive-type AF system for AF cameras, divides the image viewed through a predetermined distance measuring zone (e.g., a focus detection zone in the case of an AF camera) into two images via a pair of separator optical systems, to be respectively formed as two separate images (right and left images) on a corresponding pair of line sensors (right and left sensors). Each line sensor includes an array of photodiodes (an array of photoelectric conversion elements). Each photodiode converts the received light into an electric charge and accumulates (integrates) the electric charge. Thereafter, the accumulated electric charges are read out of each photodiode, in order, as a pixel signal (voltage). In accordance with the pixel signals output from the pair of line sensors, a predetermined distance measuring process is performed to obtain data such as the amount of defocus or the object distance that is necessary for bringing the object into focus. However, in the case where this passive-type distance measuring apparatus uses a distance measuring sensor which converts the incident light into an analog pixel signal, an accurate calculated distance value/amount of defocus cannot be attained by merely converting an analog pixel signal output from the distance measuring sensor into a digital signal and using the digital signal in a predetermined distance measuring calculation if the brightness and/or the contrast of an object is low due to, for example, the resolving power of a low brightness portion of the digital signal being low, so that the accuracy of measurement deteriorates.
It is an object of the present invention to provide a distance measuring apparatus in which the accuracy of measurement can be increased and the time necessary for measurement can be shortened.
To achieve the object mentioned above, according to the present invention, a distance measuring apparatus is provided for a camera having a plurality of distance measurement zones, including a light receiving device having a plurality of light sensors, wherein each of the light sensors receives light from an object in each of the distance measurement zones and outputs an analog pixel signal corresponding to the amount of light received; an A/D converter which converts each of the analog pixel signals into digital pixel data; a logarithmic transformation device which performs logarithmic transformation on each of the digital pixel data at a predetermined logarithmic transformation range to obtain a first transformation data; a determination device which determines a contrast state of each of the distance measurement zones based on the first transformation data in each of the distance measurement zones; a calculation device which calculates an average relative object brightness based on the first transformation data in each of the distance measurement zones; and a control device which obtains a second transformation data by performing logarithmic transformation again via the logarithmic transformation device on each of the digital pixel data in the distance measurement zone, which has been determined by the determination device as having a low contrast, by utilizing a predetermined object brightness range, including the average relative object brightness in the distance measurement zone, as a logarithmic transformation range.
Preferably, the calculation device further calculates distance measurement based on the first transformation data in the distance measurement zone which has been determined by the determination device as not having a low contrast, and the calculation device further performs distance measurement based on the second transformation data in the distance measurement zone which contrast has been determined by the determination device as having a low contrast.
Preferably, the calculation device subtracts the first transformation data from a predetermined reference value to thereby obtain a converted value, in order to obtain an average relative object brightness of each of the distance measurement zones by averaging the converted value per distance measurement zone.
Preferably, the determination device obtains a difference between a maximum value and a minimum value of the first transformation data in each of the distance measurement zones, and determines that a contrast of distance measurement zones is low when the difference is equal to or smaller than a predetermined value.
Preferably, the control device instructs the logarithmic transformation device to perform a logarithmic transformation in a logarithmic transformation range narrower than the predetermined logarithmic transformation range.
Preferably, at least one memory is provided for storing each of the digital pixel data, the first transformation data and the second transformation data.
According to another aspect of the present invention, a distance measuring apparatus is provided for a camera having a plurality of distance measurement zones, including a light receiving device having a plurality of light sensors, wherein each of the light sensors receives light from an object in each of the distance measurement zones and outputs an analog pixel signal corresponding to the amount of light received; an A/D converter which converts each of the analog pixel signals into digital pixel data; a logarithmic transformation device which performs logarithmic transformation on each of the digital pixel data at a predetermined logarithmic transformation range to obtain a first transformation data; a determination device which determines a contrast state of each of the distance measurement zones based on the first transformation data in each of the distance measurement zones; a calculation device which calculates an average relative object brightness based on the first transformation data of the distance measurement zone which has been determined by the determination device as having a low contrast; and a control device which obtains a second transformation data by performing logarithmic transformation again via the logarithmic transformation device on each of the digital pixel data in the distance measurement zone by utilizing a predetermined object brightness range, including the average relative object brightness in the distance measurement zone, as a logarithmic transformation range.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-247764 (filed on Aug. 17, 2000) which is expressly incorporated herein by reference in its entirety.